1. Field Of The Invention
This invention relates to apparatus and means for minimizing occupant injury and vehicle damage during rear end collisions. In particular, this invention relates to structures for attachment to the rear of a vehicle prone to collision by another vehicle from the rear, for the purpose of absorbing and dissipating the impact energy of the collision. The subject of this invention is the mounting of such structures to the rear of the vehicle.
2. Description Of The Prior Art
Highway maintenance activities, particularly on a high speed or heavily traveled highway, pose a high risk of danger to both motorists and maintenance workers. For this reason, slow moving or parked trucks with raised signs or brightly flashing lights are frequently used to alert the public to the closing of lanes where maintenance is being done. Despite the signs and lights, however, inattentive motorists still occasionally crash into the backs of such trucks, causing severe damage to one or both vehicles and injury to maintenance workers and vehicle occupants.
The problem has been lessened significantly by the use of deformable structures mounted to the rear of the truck (which may thus be termed a carrier vehicle), the structures containing materials capable of absorbing the energy of an impact by compression. Such materials have included foams, fibrous materials and open cellular structures.
The deformable structure to which the present invention is addressed is a rectangular box, the interior or which consists of an array of elongate cells extending in the longitudinal direction of the carrier vehicle and thus the expected direction of impact, such that the cell walls absorb and dissipate the energy of impact by collapsing longitudinally. The box is in fact divided longitudinally into a series of sections, the cells in each section arranged in bundles with voids in between, and the proportion of bundle area to void area and hence the energy absorbing capacity of each section increasing as the distance between the section and the rear of the carrier vehicle decreases. One method of achieving density variations is by grouping these cells in discrete bundles with voids between adjacent bundles. The size of the bundles increases and the void space accordingly decreases as the sections successively approach the carrier vehicle, with the forwardmost section (i.e., that closest to the carrier vehicle) containing no voids at all but rather a continuous array of cells. The result is a graduated resistance upon impact, which lessens the jolt of the impact and hence the severity of damage to the vehicles and injury to the occupants.
In addition to being deformable upon impact, the structure must be sufficiently light for towing by the truck or vehicle upon which it is mounted without severely increasing the power requirements of the vehicle or shifting its center of gravity. The structure is accordingly constructed of lightweight materials. Unfortunately, such structures are fragile and therefore highly susceptible to vibrations encountered during transit of the vehicle to and from the job site, notably the vibrations which occur as the vehicle passes over potholes and uneven road surfaces, and as the vehicle backs out of driveways. These vibrations frequently cause localized crushing of the cell structure, and opening of the bonds or welds used to secure the cell ends to the bulkheads separating the sections.
This problem has been alleviated somewhat by the addition of tension cables stretched diagonally between the front and rear ends of the structure. Typically, crossed cables extend from the bulkhead at the rear of the forwardmost section through the void spaces (which are aligned in the remaining sections) to the rear wall of the structure to enhance the rigidity of the sections. The front end of each cable is secured through connecting hardware to a bolt which extends through the entire length of the forwardmost section (which contains a continuous array of cells) and extends through the front wall of the structure, further bolting the front wall to a frame by which the structure is mounted on the vehicle itself. By passing through the cells in the longitudinal direction, the bolt unfortunately creates further localized vibrations in the cells immediately adjacent to it, causing high stress concentrations and weakening of the cell structure.
Additional bolts which do not support cables are generally present to further secure the structure to the mounting frame. Like the cable-supporting bolts, these bolts pass through the entire length of the forwardmost section of the structure, and vibrate during vehicular motion to cause disintegration of the cell network immediately adjacent.
A further source of destructive vibration are the brackets by which the frame, to which the structure is mounted, is secured to the rear of the vehicle. The vibrations, of course, are transmitted through the frame to the cell structure in general, with the greatest destructive effect occurring in the forwardmost section around the mounting bolts passing therethrough.
Since the impact absorption structures are generally unwieldy and are mounted at one end only, vehicular vibrations are magnified as they are transmitted to the structure, and the combination of these stress concentrations and weak points renders the entire structure particularly susceptible to disintegration before and between uses.
The disintegration problem is aggravated even further by the use of rotatable mountings which permit the structure to be tilted upward by as much as 60.degree. from the horizontal. Tilting is done when the apparatus is not in use, for purposes of maneuvering the vehicle and discouraging persons from sitting or placing objects on the structure. At high tilt angles, vehicle vibrations cause even greater disintegration of the cells in the regions surrounding the bolts.